class Less::Node::Expression

def to_sass_tree
  if first.is_a?(Array)
    val = map {|e| _to_sass_tree(e)}.inject(nil) do |e, i|
      next i unless e
      Sass::Script::Operation.new(e, i, :comma)
    end
  else
    val = _to_sass_tree(self)
  end
  val.options = {}
  val
end